1. Field of the Invention
This invention relates to an image forming apparatus such as a copying machine, facsimile machine, printer, and the like, and more particularly to an image forming apparatus having a function for controlling toner density.
2. Discussion of the Background
In developing methods using powder toner, particularly in a developing method using a one-component type developer including a toner as a main-component (hereinafter referred to as a one-component developing method), various methods for charging the toner have been proposed. For example, Japanese Laid-Open Patent Application No. 4-184462/1992 discloses a technique which improves the charging-property of toner by activating movement of the magnetizable toner by rotating a rod-shaped developer regulating member disposed adjacent to a developing sleeve which serves as a toner bearing member. However, in this technique, charging of the toner is mostly performed by the contact charging between the developer bearing member and the toner, and therefore uniform charge of the toner cannot be obtained.
In contrast, in a developing method using a two-component developer composed of a carrier and a toner (hereinafter referred to as a two-component developing method), toner can be more stably charged and fed when compared to the toner of the one-component developing method. Therefore, the two-component developing method is widely used for middle-copying-speed type and high-copying-speed type printers, copying machines and the like.
In the aforementioned two-component developing method, only the toner is consumed in the image developing process, and therefore a mixing ratio of the toner and the carrier varies. Therefore, it is necessary to maintain the mixing ratio of the toner and the carrier within a fixed range to stably obtain images having good image qualities. In other words, if the toner density (the weight ratio of the toner to the developer) changes, an amount of toner supplied to a developing area and an amount of charge of the toner changes, resulting in variations in image quality. More specifically, when the toner density is relatively low, the toner cannot be sufficiently supplied to the developing area. In addition, the amount of charge of the toner increases resulting in deterioration of the developing ability of the developer. On the other hand, if the toner density is relatively high, the toner is oversupplied to the developing area, or the toner tends to adhere to the image bearing member due to insufficient charging of the toner caused by a decrease in the probability of contact of the toner with the carrier. Accordingly, the toner undesirably adheres to a non-image part of the image bearing member, resulting in fouling of the background of an image (hereinbelow referred to as background fouling).
In order to control the toner density, a toner supply control method has been proposed in which a toner supplying device controls the toner supply based on data of a toner density in a developing unit detected by a toner-density detecting device using a permeability measuring sensor or the like. However, for example, in a low cost and light-duty copying machine, the copying machine is often left without being used for extended periods of time and, therefore, the developer tends to be left for long periods of time without being used. If the developer is left for a long period of time without being used, the charge to the carrier and the toner of the developer in the developing unit is naturally discharged resulting in a decrease of the amount of charge of the carrier and the toner. When an image forming operation is performed after leaving the apparatus unused for a long time, problems such as toner scattering and background fouling caused by the decrease of the charging amount of the toner tends to occur.
In addition, because of the decrease of the charging amount of the developer including the carrier and the toner, repulsion of the developer decreases or air enters the developer naturally, resulting in a decrease in the bulk of the developer. The aforementioned permeability sensor detects a distance between the carrier (which is magnetic) and the sensor. When the amount of toner decreases, the carrier is close to the sensor, and therefore the toner density is judged to be low by the sensor. In addition, however, when the carrier is close to the sensor because of a decrease in the bulk of the developer, the sensor erroneously detects that the toner density has decreased, although the toner density has not varied. Since the toner supplying device supplies toner on the basis of the data detected by the sensor, the toner density in the developing unit increases, resulting in occurrence of problems such as toner scattering and background fouling.
Such a problem occurs when the toner density is falsely detected with a toner density detecting devices and can occur not only in a permeability sensor type device, but also with other toner density detecting device having a construction which outputs false detected data from other than a permeability measuring sensor. For example, other toner density detecting devices detect the toner density by methods influenced by a decrease of the charge amount or the bulk of the developer which tend to occur when the image forming apparatus is left without being used for a long period of time.